Witches broom tree disease

Witches’ Brooms on Trees

A willow branch was recently submitted to the Plant Disease Clinic that had a dense cluster of twigs, a symptom referred to by plant pathologists as a “witches’ broom”.

In medieval times, mysterious and unexplainable occurrences were often blamed on witchcraft. Brooms during this time were made of bundles of twigs. The term witches’ broom comes from the German word Hexenbesen, which means to bewitch (hex) a bundle of twigs (besom).

Witches’ brooms occur on many different woody plant species, including deciduous trees such as hackberry, maple, and willow, and conifers such as pine and spruce. There may be only one broom in a tree, or they may be many scattered throughout the tree. In some cases, the brooms are quite large in size and are easily spotted. In others, they are small and well-hidden.

A number of stresses, both biological and environmental, can lead to the formation of brooms. Organisms such as fungi, phytoplasmas (bacterial-like organisms), mites, aphids, and mistletoe plants can cause abnormal growth when they attack a host tree. Environmental stresses that injure the growing points of branches can also trigger the formation of brooms. Some brooms appear to be caused by genetic mutations in the buds of the branches. Unlike brooms caused by living organisms, there is usually just one broom per tree when the cause is a genetic mutation.

Pinpointing the cause of a witches’ broom can be difficult, especially if the formation is related to an environmental factor. Analyzing the plant tissue for infectious agents such as phytoplasmas requires specialized testing that can be costly. The cause of the witches’ broom on the willow sample sent to the Plant Disease Clinic was not identified with certainty.

Witches’ broom, cause unknown, on willow.

This article originally appeared in the 2/23/2005 issue.

Witches’ Broom in Trees: What, Why & What To Do

It’s almost Halloween so it’s a good time to learn about witches’ brooms, but sadly not the flying kind.

The household brooms we use for sweeping are called brooms because they were once made from broom itself—the genus Genista, in the pea family, that includes many species of yellow-flowering plants with small, twiggy growth. These are the old-fashioned brooms that are made from bundles of small, irregular, flexible twigs bound to a long handle, not today’s corn or palm fiber brooms.

Nowadays, you may find similarly strange masses in your shrubs and trees, or even weird-looking growth on your rosebush. If you do, it might be witches’ broom.

How can I identify witches’ broom?

The more you look at plants, the more you see. Is that mass of twigs up in the crown of a deciduous tree a squirrel’s nest? Are those strange buds normal? Is that dense cluster of needles on the pine tree just vigorous growth? Maybe, or it might be witches’ broom.

Common signs of a witches’ broom include:

  • a bunch of twigs or small branches growing from a central point (often growing around a larger branch)
  • a ball-shaped dwarf plant growing in the tree
  • an unusually dense cluster of foliage or needle growth

Read on for more information about this weird and fascinating condition that can be found on conifers, deciduous trees, vines, shrubs, and perennials.

What causes witches’ broom?

There are a number of things that can cause a witches’ broom to develop. Some tree species are susceptible to pathogens that cause a witches’ broom to grow in response, some common insect pests can cause it, and sometimes it’s just a genetic mutation (called a “sport”).

  • Phytoplasmas are parasitical bacteria that make their home in the phloem, or food-transporting network of a tree. They’re usually transmitted to the plant tissue by an insect. Phytoplasmas can damage a wide range of plants, from wine grapes to coconut palms, by interrupting the flow of food and energy to the host plant.
  • Insects, such as aphids and mites, can carry a pathogen that causes a witches’ broom to grow, and those pests can pass it along to the plant they’re infesting. For example, one of the symptoms of rose rosette disease is a witches’ broom; it’s caused by a virus that is borne by mites. Rosebuds and stems become stunted, grow many tiny leaves and excessive thorns (often they’re a strange red color), and the rosebuds do not open into flowers.
  • Dwarf mistletoe is a parasite that infects pine species and other conifers. Dwarf mistletoe is not the green bunch that you hang in a doorway; it’s a small, leafless plant that grows on trees by taking water and nutrients from its host. The damage to tree tissue by the parasitic plant results in the deformed growth that makes a witches’ broom. For infected trees, this damage is often fatal as the irregular growth girdles branches and the mistletoe siphons the tree’s energy.
  • Powdery mildew, a common fungus, has been known to cause oak trees (sp. Quercus) to develop witches’ brooms. A long period of rainy but warm weather can foster the growth of powdery mildew, and a witches’ broom is the tree’s response to an infection from the fungus. The strange appearance of leaf and stem growth can be the result of reduced photosynthesis where powdery mildew has covered leaf surfaces.

Witches’ broom produced by effects of an eriophyid mite and/or an associated powdery mildew producing fungus. Photo courtesy of
Whitney Cranshaw, Colorado State University, Bugwood.org

Which trees are more likely to develop a witches’ broom?

Pretty much any tree can develop a witches broom under the right conditions. We often see witches’ broom in Norway spruce here in central New Jersey, as well as in American beech. Other common species that develop witches’ broom in response to pathogens, insect pests, or stressful environmental conditions include:

What about genetic causes?

Genetics can cause normal twig and needle growth to produce witches’ brooms, or “sports,” on conifers. The altered plant tissue often grows to form a ball of dwarfed needled twigs and branches in the tree’s crown. A notable difference between pathogen and parasite caused witches’ broom and a genetically caused witches’ broom is the health of the host tree. Conifers with genetic witches’ broom are healthy, and the sport itself looks like a tiny version of the species.

These naturally occurring miniatures are the source of some of the dwarf cultivars of conifers found in nurseries, such as ‘Blue Star’ juniper. This, and other, cultivars were developed by plant hunters and scientists who climbed trees to collect witches’ brooms, and then used cuttings from them to grow new plants that were healthy and carried the genetics for dwarfism.

Should I worry if I see witches’ broom in my tree?

The short answer is … it depends!

Most witches’ brooms are not fatal, just disfiguring. For example, irregular bud growth or overly dense areas on trees are unlikely to seriously injure the tree.

On the other hand, rose rosette disease is fatal and can spread to nearby roses as mites carry the virus from plant to plant. The current recommendation is to remove all parts of any affected roses (including roots and fallen leaves) and dispose of them in a sealed plastic bag.

Depending on the plant and the cause, witches’ broom may be:

  • part of one year’s growth response and could be supplanted by normal growth in the future,
  • shed when leaves fall and not recur the next year, or,
  • as in the case of genetic creation, it could be just a part of an otherwise healthy conifer.

If you’re not sure what you’re looking at or what caused it, call the experienced tree care professionals at Organic Plant Care to help diagnose the problem. We can tell you if it’s benign, damaging, or fatal.

If your witches’ broom is caused by insect pests, we can also explain what controls to use to prevent it in the future.

Large witches’ broom in a douglas-fir

What should I do if my tree has a witches’ broom?

If the aesthetic appearance of your shrubs or trees is a concern, the first thing to do is to prune off the affected portions. Depending on the cause of a witches’ broom, removing affected branches, twigs, buds, or leaves may be all you need to do. Be sure to make your pruning cut at a proper location along a branch or twig before the affected portion.

Discard the cuttings right after pruning, rather than leaving them to decay into compost or leaf mold. This will reduce the availability of potential sources of reoccurrence, such as fungal spores or insects.

Clean your tools after using them to cut away a witches’ broom. Disinfecting a tool after each cut can limit the likelihood that you will transfer a damaging organism to an uninfected branch when you prune it.

If you’re not an experienced pruner, or if the witches’ broom is up in the crown of a mature tree, call us to handle the job. We know not just how to remove the affected portions (and have the training and equipment to do it safely), but whether or not it will solve the problem.

Powdery mildew damage, for example, can be removed by pruning, but pruning won’t prevent powdery mildew from coming back. Aphid infestations are another example; non-toxic sprays to kill aphids need to be applied at particular times and pruning isn’t required.

Our tree care professionals will also be able to tell you if external conditions are helping the growth of witches’ broom, as trees and shrubs under stress are more susceptible to developing it. Sometimes, removing or altering elements around your trees can increase their health and vigor, as well as their resistance to the causes of these growths.

HAPPY HALLOWEEN!

Witches’ broom has a long and interesting history. Once they were thought to be the product of witchcraft, with the masses in trees being thought of as both places that witches retreated to for rest and the vehicle for their flying. A witch’s broom cut from a tree might become a protective talisman, hung to keep animals safe.

We know now that folklore is not a substitute for science and scientific tree care, but the name has stuck and it connects us to our distant past.

Scientists seek cure for devastating witches’ broom disease of the chocolate tree

At one stage of its lifecycle, Moniliophthora perniciosa takes on the form of enchanting pink mushrooms that seem to come straight from a fairytale (see picture). For the chocolate tree, however, M. perniciosa spells trouble. These mushrooms are filled with millions of spores that, once released, can enter a susceptible chocolate tree through surface wounds and tiny gaps called stomata and slowly kill the tree. Because infected trees develop bizarre green outgrowths that resemble brooms, the disease is known as witches’ broom disease. Two to three months after infection, the brooms turn brown and begin to perish. The fungus then completes its lifecycle by once again giving rise to clusters of spore-producing mushrooms. There is no known cure for this devastating disease.

In 2000, a team of scientists led by Gonçalo Pereira of the Universidade Estadual de Campinas in Brazil initiated the Witches’ Broom Genome Project, with the long-term aim of developing a cure for witches’ broom disease. A study to be published in The Plant Cell represents the culmination of their research to date. The team used a technique known as dual RNA-seq analysis to monitor the interaction between the M. perniciosa fungus and the chocolate tree. This technique allows scientists to reconstruct the battlefield between the chocolate tree and the fungus in unprecedented detail, by providing a readout of genes that are affected in the plant and the fungus during the course of witches’ broom disease. “Knowing the molecular and physiological basis of a disease is an important step towards developing effective control strategies,” says study author Paulo Teixeira. Using healthy plants as a reference point, the scientists identified 1,967 genes that exhibited unique activities in the green broom structures of infected chocolate trees. An analysis of these genes showed that fungal infection triggers massive changes in the metabolism of the chocolate tree. Additionally, the scientists discovered 8,617 fungal genes that were active in green brooms. Using the Witches’ Broom Disease Transcriptome Atlas, a publicly available online tool developed by Pereira’s team to support studies of witches’ broom disease, the scientists identified 433 fungal genes that were particularly active in green brooms. Many of these genes encoded proteins with presumed functions in the fungal disease mechanism. Study author Daniela Thomazella explains that “The discovery of sets of fungal genes that are most likely involved in pathogenicity paves the way for the development of targeted treatments of the disease.” Indeed, the authors are already using the results of this study to develop a novel fungicide that specifically targets M. perniciosa. In addition to increasing our knowledge of a devastating tropical disease, lead scientist Pereira maintains that this work provides an important basis for future studies that aim to improve agricultural sustainability and global food security.

Witches’ Broom – Trees

An example of a witches’ broom

Witches’ brooms are characterized by a proliferation of shoots growing close together. The shoots are usually shorter, stockier, and have an upright but more compact growth habit than normal. Witches’ brooms may be caused by fungal, viral, or mycoplasma-like organisms (MLOs). Eriophyid mites, mistletoe, environmental damage, or a mutation in vegetative cells may also cause witches’ brooming. In most cases, the causal agent kills a growing point and results in the prolific growth of side shoots. The growth around the witches’ broom may become less vigorous, indicating that the witches broom may divert nutrients from other parts of the plant. When witches’ brooms are caused by mutation, horticulturists sometimes propagate them for breeding of dwarf plants.

Mycoplasma-like organisms (MLOs) are also called phytoplasmas. They are related to bacteria, lack a rigid cell wall, and have an amoeba-like shape. MLOs appear to colonize in the sap conducting tissue (phloem) and damage the tissue by interrupting the sap flow. Diseases caused by MLOs are elm yellows, ash yellows, and bunch diseases of walnut. Witches’ brooming, chlorosis, and general decline are symptoms of these diseases. MLOs may also be responsible for witches’ brooming in lilac, dogwood, willow, apple, black locust, honeylocust, papaya, peach, and sassafras.

Witches’ brooms can be a symptom of fungal or viral infection. The fungus Ascomycetes causes witches’ broom of cherry. The powdery mildew fungus, Sphaerotheca lanestris, may cause witches’ brooms on live oak, willow oak, and ninebark. The fungus, Gymnosporangium nidus-avis, causes juniper broom rust. Other fungi cause witches’ brooming primarily in evergreen plants.

When witches’ brooming is noticed, prune out the affected parts, if possible. When fungi, virus or mycoplasma-like organisms are responsible for witches’ brooms, the disease may have spread throughout the tree, so that pruning may not provide control.

Cherry (Prunus spp.)-Witches’ Broom

See:

Peach (Prunus persica)-Leaf Curl

Cause Taphrina wiesneri, on cherry, T. confusa on Chokcherry, and T. flavorubra on Western Sand Cherry, fungi. Ascospores from diseased leaves fall on buds and, upon germinating, penetrate the branch, stimulating it to abnormal growth year after year. Once a branch is infected, it will always remain so, and the leaves on the “broom” will be diseased every year. In Oregon, the disease is only on sweet cherry trees.

Symptoms Large, broom-like tufts develop on branches. They are easily distinguished at blossom time: they have few flowers and become leafy earlier than normal branches. Brooms do not bear fruit. At the base of the witches’ broom is a common stem, which may be much thicker than the branch to which it is attached. Long, slender branches grow from this stem. Affected leaves somewhat resemble peach leaves affected by peach leaf curl-thick, reddish, with a white growth of fungus (asci) on the undersurface.

Cultural control

  • Prune out the witches’ brooms. Trace the main stem of the “broom” to where it grows from the normal branch, then cut the branch at least 12 inches below that point.

Chemical control Make an application at delayed dormant growth stage.

  • Rex Lime Sulfur Solution (28%) at 8 to 10 gal/100 gal water. 48-hr reentry. O

Reference Shoji, T., and Sato, K. 1980. Control of witches’ broom on cherry trees caused by Taphrina wiesneri by pruning of the diseased twigs and fungicidal application. Annual Report of the Society of Plant Protection of North Japan 31: 95-97.

Witches’ Brooms

The term “witches’ brooms” may bring up images of Halloween transportation devices, but it also describes a type of growth disorder in plants. Witches’ brooms are dense masses of shoots arising from a single point on an otherwise normal branch. The shoots typically display dwarfed characteristics: slow growth, very short internodes (the space between leaves or side branches), and much smaller than normal leaves.

Witches’ brooms can be caused by a number of factors that alter normal growth including pathogens (fungi, viruses, bacteria, phytoplasmas), parasitic plants (dwarf mistletoe), and arthropods such as mites and aphids. Other witches’ brooms result from genetic mutations that arise from unknown factors.

Dense, twiggy witches’ brooms are frequently seen on common hackberry (Celtis occidentalis). Photo by Whitney Cranshaw, Colorado State University, bugwood.org.

Common hackberry (Celtis occidentalis), an adaptable North American shade tree, is frequently affected by witches’ brooms that seem to have two contributing causal agents: powdery mildew fungus and a tiny eriophyid mite. The witches’ brooms typically don’t affect the tree’s health (unless there’s a severe infestation) but can detract from its appearance. Another unsightly witches’ broom that I noticed in the Arboretum recently is common to a number of shrub honeysuckles (Lonicera) and is caused by the feeding of honeysuckle aphids, sometimes called Russian or Tatarian aphids for their Eurasian native range.

A witches’ broom caused by aphids on a shrub honeysuckle at the Arboretum (Lonicera x bella ‘Candida’, accession 3475-A). Photo by Nancy Rose.

Many conifer species, including pines (Pinus), firs (Abies), spruce (Picea), and junipers (Juniperus), are noted for producing witches’ brooms. These may be caused by diseases such as spruce broom rust (a fungus) or the parasitic dwarf mistletoe, while others are genetic mutations. The latter are of particular interest for their ornamental potential as dwarf conifers. As far back as the mid 1800s, plant propagators were selecting, propagating, and naming cultivars of dwarf conifers that arose from witches’ brooms. The slow, dense growth and proportionally petite leaves of these cultivars make them popular for rock gardens and other landscape sites where small plants are desired. Interestingly, when conifer witches’ brooms produce seeds a fairly high percentage of the resulting seedlings may also have dwarfed characteristics. For a fascinating look at the etymology (yes, witches are involved!), history, and propagation of conifer witches’ brooms, read this 1967 Arnoldia article by longtime Arboretum propagator Al Fordham.

Blue Star single-seed juniper (Juniperus squamata ‘Blue Star’) is a popular dwarf conifer that arose from a witches’ broom. Photo by Nancy Rose.

Spotting witch’s brooms

A witch’s broom lurks in a scrub pine along Estes Drive Extension. Photo by Ken Moore.

By Ken Moore
Over the years, I’ve enjoyed collecting witch’s brooms. Well, now, I haven’t actually collected them; I’ve merely spotted them, and revisit them often, quite often, since most obvious ones are along roadsides.

They are like old friends, and I smile inside every time I pass one.

My favorite witch’s broom is a fine specimen perched midway up a scrub pine, Pinus virginiana, along Estes Drive Extension between Chapel Hill and Carrboro.

Just before the holidays, I spotted one midway up a loblolly pine, Pinus taeda, along the ramp from Smith Level Road onto the U.S. 15-501 bypass. Can’t believe I had never noticed it; guess I’ve been paying more attention to vehicles on the road – not a bad thing. “Witch’s broom spotting” should not become a cell phone-like distraction while driving.

Last week, I spotted another one in a loblolly pine while walking through the pine forest of Carrboro’s Adams Tract.

A witch’s broom is an abnormal growth in a tree, usually caused by a virus or fungus. The growth is a dense mass of shoots growing from a single point, resembling an old-timey broom or strange-looking bird’s nest. There is not a lot of information on what is really going on with this plant-growth curiosity. I suspect there is simply not enough interest or concern for any young botanist to pursue a doctoral study.

I share my witch’s broom sightings with botanical garden nursery manager Matt Gocke, who is hoping to propagate some interesting-looking trees from some of the brooms. One way of producing plants with the compact, dwarf characteristics of witch’s brooms is to make grafts of the broom branches, or “scions,” onto the stems, or “stocks,” of normal-growing plants of the same species.

If the witch’s broom produces cones, then another possibility is to grow a dwarf tree from a seed collected from that abnormal growth. One such oddity is a dwarf loblolly pine growing along the north walk in the Coker Arboretum. That one is a dwarf seedling from a magnificent witch’s broom high up in one of the arboretum’s big pines. Sadly, that tree was killed by lightning two years ago. On your next walk through the arboretum, look for curator Margo McIntyre or one of her assistants to help you find that dwarf loblolly.

These rare curiosities are most frequently spotted in pine trees, though I did find one in a red cedar, Juniperus virginiana, several years ago.

More commonly spotted are the witch’s brooms in hop hornbeams, Ostrya virginiana, easily observed in winter months, when the messy looking, dense, leafless twig structures can be quite numerous on a tree, giving it a truly “bad-hair day” look. There are lots of them along the trail encircling Big Oak Woods at Mason Farm Biological Reserve.

A witch’s broom looks like a bad-hair day in hop hornbeam. Photo by Ken Moore.

Folklore is filled with stories of witches and their flying about on coarse twiggy brooms. I like the story of witches flying over trees to make brooms grow in them. I enjoy thinking about that story when walking beneath all those witch’s brooms in the Mason Farm hop hornbeams.

It’s fun to look up into trees; never know what you may find there!

Colorado Master Gardener: Witches’ broom

Vicky Barney

Witches’ broom is the name given to the strange looking knot of growth on trees and shrubs. It looks like a broom — a large number of small branches growing from one spot — and may be found on both deciduous woody plants and conifers. It is interesting to observe out in the forest but may be concerning if found close to home.

A tree or shrub may grow witches’ brooms when stressed by insects, like mites or aphids, or a plant pathogen like fungi, bacteria, viruses, phytoplasmas or by parasitic plants In Colorado conifers, the stressor is likely one of five parasitic dwarf mistletoes (Arceuthobium).

Dwarf mistletoe infects a tree by growing root-like structures under the bark and into the wood. It lives by pulling nutrients and water from the tree.

It is a slow growing organism: after several years it will develop inconspicuous flowers and produce fruit that when ripe, will explode and send a seed into the air. The seed will stick to any surface up to 60 feet away. If the surface is a susceptible tree branch, the seed will germinate and grow into the bark, spreading the infection to another tree.

Witches’ brooms caused by other stressors rarely kill the host plant and may be pruned out to improve the appearance of the tree or shrub. Dwarf mistletoe, however, can be deadly. Over time, infected trees fail to thrive and may have witches’ brooms, unhealthy looking foliage and dead branches. The trees are then susceptible to fatal problems, like pine beetle attacks.

While no viable treatment is available for infected conifers, proper management may slow or stop a dwarf mistletoe infestation. Severely affected trees should be removed and other trees pruned to remove infection from lower branches. As dwarf mistletoe is species specific, its spread may be halted by planting different tree species between infected trees.

A chemical spray may be warranted under special circumstances. Please see CSU Extension Fact Sheet No. 2.925 for more details.

For more Colorado State University Master Gardeners are available to answer questions from 10 a.m. to 1 p.m. Thursdays through the gardening season at the CSU Extension Office, 136 Sixth St. in Steamboat Springs. Contact 970-870-5241 or [email protected] with questions or to schedule a site visit.

If you find a witches’ broom in your conifer and seek pruning information, please see CSU Extension GardenNotes No. 633. If you suspect a dwarf mistletoe infestation and would like help with a management plan, please contact a professional forester, the Colorado State Forest Service or the Master Gardener program.

Vicky Barney gardens for wildlife and is a member of the Master Gardener Class of 2011.

Plant of the Week: Witch’s-broom

The University of Arkansas System Division of Agriculture does not promote, support or recommend plants featured in “Plant of the Week.” Please consult your local Extension office for plants suitable for your region.

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Plant of the Week

Montgomery Dwarf Blue Spruce is one of the best dwarf blue spruce clones grown from a witch’s-broom.

With hundreds of years of gardening history under our collective belt, we gardeners have accumulated a wealth of trophies to show off in our gardens. One broad group of these is the dwarf plants grown from naturally occurring witch’s-brooms. Witch’s-brooms, though most commonly associated with conifers such as the pines and spruces, can occur in any woody plant.

A witch’s-broom is an abnormal cluster of growth that forms somewhere among the branches of an otherwise normal-looking tree. The typical witch’s-broom is characterized by excessive branch production, short stems and usually leaves or needles that are one-fourth to one-half those found on the normal part of the tree.

The causes for these abnormalities are many but are primarily thought to be associated with a stress-induced genetic change in the genome. The stressor is sometimes an insect (usually aphids or mites) or a plant pathogen such as fungi, bacteria, viruses or phytoplasmas. Because witch’s-brooms are usually rare and widespread in nature, the pathogens causing them must not be very virulent or very easily transmitted by vectors.

Two bits of evidence provide credibility that this dwarfism is associated with a change at the DNA level. First, afflicted plants never grow out of the condition, even if grafted on normal seedlings. Secondly, this dwarfism is transmitted to future generations by seed at about the 50 percent rate.

The rate of dwarfism varies considerably. Some, such as an especially slow-growing form of hemlock, produce only one-fourth of an inch of shoot growth per year while others may make 2 to 4 inches. The growth form can also vary. While the round ball form is most common, some witch’s-brooms produce prostrate spreaders, weepers or narrow upright branching forms.

Globosum is a round-headed grafted form of Japanese black pine.

The principle characteristics of a witch’s-broom—increased branching and dwarfism—indicate that the plant’s hormonal biochemistry is being disrupted. Though this has never been illustrated as far as I know, there is probably a significant increase in the cytokinin content (a plant hormone known to promote branching), and a reduction in production or a blockage in the activity of auxin, a phytohormone needed for normal cell expansion and growth.

Witch’s-brooms have been observed for centuries. The term itself is taken from German hexenbesen—literally a hex put on a broom. In the forest, these clumps of foliage were thought to be the lair of witches, elves, hobgoblins, and mares (an evil spirit that would sit on your chest at night and cause bad dreams or nightmares). It was not until the Victorians began climbing mountains in the mid-1800s and began building rock gardens that an interest developed in propagating these dwarf plants for use in these new kinds of gardens.

Because dwarf forms can be selected from the witch’s-broom itself in the tree or from seedlings grown from a witch’s-broom, the number of potential plants is enormous. One gardener boasts on the Web of having more than 2,000 different kinds of dwarf conifers! For a dwarf form to be in the nursery trade, the plant must make enough growth to produce a salable plant in at least a decade. Because of their slower growth rate and because many must be grafted, dwarf conifers are a part of the specialty nursery trade and tend to be pricey. Dwarf forms are especially common in pines, spruce, arborvitae and Chamaecyparis.

Dwarf conifers are ideal landscape plants because their small stature reduces the likelihood they will outgrow their allotted space. Because they are conifers they do best in at least six hours of direct sunlight a day and in a reasonably fertile, well-drained soil. Because they are so slow-growing, they must not be crowed by normal plants that will take advantage of their slowness and shade them out.

By: Gerald Klingaman, retired
Extension Horticulturist – Ornamentals
Extension News – February 12, 2010

The University of Arkansas Division of Agriculture does not maintain lists of retail outlets where these plants can be purchased. Please check your local nursery or other retail outlets to ask about the availability of these plants for your growing area.

Signs And Symptoms Of Witches’ Broom On Trees And Shrubs

Ever seen those odd-looking, broom-like distortions in a tree? Perhaps it’s one of yours or in a tree nearby. What are these and do they cause any harm? Keep reading to find out more about the signs and symptoms of witches’ broom disease.

What is Witches’ Broom Disease?

Witches’ broom is a symptom of stress found in woody plants, mainly trees, but also affects shrubs as well. This includes deciduous trees and shrubs as well as conifers and evergreen varieties. The stress results in a deformed mass of twigs and branches, which appear broom-like in appearance. As brooms were once fashioned together from bundles of twigs and since witches were presumed to be responsible for anything unusual, these abnormalities became known as witches’ brooms.

What Causes Witches’ Brooms?

Witches broom disease is not caused by witches, however. They’re actually caused by stress that is brought on by pests or disease. This includes anything from mites, aphids, and nematodes to fungi, viruses, and bacterial organisms (phytoplasmas).

In addition, parasitic plants like mistletoe, which cause stress to host trees, can lead to the formation of witches’ broom. Environmental factors may also be to blame and some are caused by genetic mutations.

Generally, the type of tree/shrub is a good indicator of its causal agent. For instance, pine brooms are commonly caused by rust fungus. Fungal infections can also affect cherry trees and blackberry bushes, forming broom growth. Peach trees and black locust can be affected by viruses that can result in witches’ brooms. Hackberry trees can get brooms as well, and these are normally caused by both fungus and mites.

Mites can also be responsible for witches’ broom in willow trees. Aphids are generally to blame for these deformities in honeysuckle shrubs, while phytoplasmas lead to the disease in ash and elm trees.

Witches’ Broom Signs and Symptoms

Witches’ broom can be easily identified by the dense clusters of twigs or branches, which grow from a central source—resembling a broom. It is best seen on deciduous trees or shrubs when they are not in leaf. While needled trees, like pines, may consist of denser needles.

There may be only one broom seen, or in some cases, there may be many. Some may be large, while others may appear quite small and less noticeable.

Witches’ Broom Treatment

Witches’ broom can occur for several months to several years, and while it may be unsightly to some people, it really poses no serious threat to the tree or shrub affected. There is currently no cure or treatment for witches’ broom. You can, however, prune out the broom growth several inches below the point of its formation, if desired.

What Is Witches’ Broom & How Does It Affect Cacao?

Like any other plant, cacao is vulnerable to pests and diseases. One of the most damaging infections is witches’ broom, an aggressive fungus that can kill the tree and wipe out whole farms. In the 1980s, the disease devastated Brazil’s cacao production and it’s never far from producers’ minds.

Let’s take a look at what witches’ broom is and how cacao farmers can handle it.

Lee este artículo en español ¿Qué es la Escoba de Bruja y Cómo Afecta al Cacao?

Arataca, Bahía, Brazil, one of the areas hit hard by witches’ broom in the 1990s. Credit: Tuta Aquino

A Brief History of Witches’ Broom

Today, most of the world’s cacao is produced in Côte d’Ivoire and Ghana. But as recently as the early 1990s, Brazil was also a major producer. Then a fungus ran rampant through the nation’s cacao plantations and the industry was decimated. The fungus responsible was Moniliopthora perniciosa, more commonly known as witches’ broom (or, sometimes, witch’s broom).

Cacao producers were familiar with the fungus, which is indigenous to the Amazon basin. But the outbreak in the Bahía region spread more rapidly than ever seen before. It’s reported that cacao output fell from 390,000 metric tons in 1988 to 123,000 metric tons in 2000.

There is some evidence that the outbreak of witches’ broom in Bahía was intentional. Some have even termed it bioterrorism. Tuta Aquino is a cacao farmer at Vale Potumuju in Bahía. He says, “I’m convinced it was brought to the region and deliberately spread in our plantations.”

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Cacao pods spoiled by witches’ broom. Credit: Tuta Aquino

How Witches’ Broom Attacks

Witches’ broom is a hemibiotrophic fungus, meaning that it functions in two stages. In the biotrophic stage, the tiny fungus spores enter healthy cacao trees through surface wounds or small gaps.

The fungus extends tendril-like mycelia in between the cells of the plant and it feeds on the living tissue. The infected shoots morph into swollen spindly stalks, or the “brooms” that the fungus is named for. The fungus diverts the plant’s energy away from effective growth and eventually causes cell death.

The necrotrophic phase takes place two to three months after infection. The mycelia start to feed on the dead plant cells and the brooms change from green to brown, commonly known as dry brooms. Infected pods show rotten spots and they’re likely useless for consumption or to grow new trees.

Finally, when temperature and humidity levels are at the right point, dramatic pink basidiocarps (mushroom fruiting bodies) form and release new spores. In the humid Amazonian climates, conditions are perfect for them to flourish and spread quickly.

The broom shaped stalks of an infected tree. Credit: Tuta Aquino

Tackling Witches’ Broom

When witches’ broom broke out in Bahía in the early 1990s, Brazilian cacao agency CEPLAC advised producers to raze their crops. Entire forests were destroyed to prevent the spread of the fungus, and farmers took out loans to cover the losses of their crops. This left many in deep debt. The Financial Times reports that the collapse of the industry cost an estimated 200,000 jobs.

Because there is no cure or effective treatment for witches’ broom, producers must focus on prevention. Tuta tells me that “the pruning is the main thing you have to do… All the dead leaves, all the new sprouting of the brooms, you clip them out.”

Cacao pods infected by witches’ broom. Credit: Tuta Aquino

Many producers treat pruned material with Tricovab, a competitor fungus formulated by CEPLAC and released in 2013. But some producers are cautious about the effectiveness of the product. Tuta says that “’s research is done at their farm with 500 plants… that’s not real life.”

Witches’ broom has proved resistant to copper-based fungicides, though there is some evidence that these fungicides reduce the number of affected pods.

Using Tricovab and taking on labour to effectively monitor and prune trees adds to the cost of cacao production. And even if a producer is vigilant, the spores spread so easily that crops can quickly become reinfected. Tuta says, “If across the river your neighbor doesn’t do it, the wind blows it over, and all of your work .”

Cacao beans drying. Credit: Miguel Regalado

Genetic Resistance

So what can producers do to protect their crops from witches’ broom? For farmers starting from scratch, choosing varieties with genetic resistance is an important consideration. In the 1970s, a high-yield strain of cacao resistant to witches broom was developed in Ecuador. The problem is that this CCN-51 strain has a reputation for subpar flavour. For producers looking to sell to specialty chocolate-makers, this is a real concern.

But Nicaraguan cacao producer and exporter Gustavo Cerna pushes back against the reputation. He tells me, “If you don’t have good post-harvest practices, quality isn’t there” but that “as with coffee, you can bring out good attributes with good post-harvest practices”.

He makes a comparison with the coffee variety Catimor, which is known for its resistance to coffee leaf rust more than its cup quality. He points out that in the 2017 Nicaragua Cup of Excellence auctions, a Catimor scored 90.5 points.

Gustavo says that it’s too early to conclude that CCN-51 can’t taste good, telling me, “I don’t think we should be gauging the quality of the product because we haven’t tested or experimented with any of the harvesting or post-harvest practices.”

Cacao beans in a fermentation box. Credit: Tuta Aquino

Even if a producer chooses to plant a resistant variety, maintaining genetic purity is a difficult task. Cacao is “self-incompatible”, meaning that it requires cross-pollination from a different cacao plant to produce fertilised seeds. This is good for genetic diversity, but bad for farmers who want to maintain genetics for quality, productivity, or resistance purposes. Therefore, some producers choose to use cloning to ensure genetic traceability.

Farmers can spend less money upfront by planting cacao seeds, but will quickly lose control of the genetic purity of their orchard because of cross-pollination. Or they can spend more money for certified clones which will provide stability over the long run.

Gustavo tells me that a standardised orchard simplifies life for the farmer. “You will be able to have uniform pruning and uniform fertiliser management,” he says. This means production and processing is more consistent and the final product is more uniform.

Cacao trees that have cross-pollinated will grow at different rates and sizes and mature at different times. “If you have a salad mix of different varieties, it just becomes harder and more expensive, and it’s not productive in the long term,” Gustavo says. And of course, any genetic resistance to witches’ broom and other diseases is likely to be reduced.

A view from Tuta Aquino’s farm, Vale Potumuju, in Bahía, Brazil. Credit: Tuta Aquino

Good Practices For Prevention

In Brazil, many producers used the traditional cabruca system, which recreates the natural conditions of the Amazon basin. “The cabruca system is where you plant the cacao trees under the shade of forest trees,” Tuta tells me.

It’s ideal for cacao and beneficial for biodiversity. But what is good for cacao is also good for witches’ broom. When the fungus took hold in Bahía, the cabruca system was seen as a liability.

Today, cacao producers are adapting their cultivation practices. Gustavo tells me that in the eastern parts of Nicaragua, where land is cheap and rainfall plentiful, farmers are planting with more space between trees to mitigate the risk of infection and spread. “They’re not allowing the plants to interlapse with one another,” he says, “so there is always a dry environment around them. The main drawback is that you cover more land.”

Healthy cacao pods on a tree. Credit: Miguel Regalado

Gustavo is taking a different approach. “We’re looking to plant cacao in the driest area we can find in Nicaragua. As long as the soil’s good and we have cheap access to water, we’ll plant cacao there,” he says.

This method reduces risk because fungus spores germinate in humidity. Witches’ broom may still be present, but will stay dormant without the right level of moisture. Irrigation allows farmers more control over the amount of water the plants receive.

Gustavo tells me that small-scale farmers need to cooperate since one mismanaged farm can infect all nearby ones. He also says that governments or other agencies need to provide technological innovation and support to producers to tackle witches’ broom, and these agencies and farmers need financial support.

A cacao tree affected by witches’ broom. Credit: Tuta Aquino

Preparing For Future Outbreaks

Preventative methods add costs to production. In relation to this, Gustavo tells me that the world’s top cacao-producing region, West Africa, is at risk of an outbreak of witches’ broom. “The risk is high. You have a lot of smallholder farmers that are not following good practices, with very, very old trees… There’s not a lot of innovation happening, so the risk is huge,” he says.

Producers in Côte d’Ivoire and Ghana don’t have the resources to invest in new crops and implement best practices. Gustavo acknowledges this, saying that “the return to farmer percentage of the FOB is really low in West Africa. It’s extremely low.” This means that the producers who are already seeing the least return on their work are the most vulnerable to devastation by witches’ broom.

“We’re preparing for disaster,” Gustavo says, meaning that Nicaragua is readying its production to fill the gap that a collapse in the West African cocoa industry would bring.

Cacao is also making a resurgence in Brazil. Tuta tells me that it simply isn’t profitable to grow cacao for commodity, but that with a growth in bean-to-bar chocolate-makers as well as new focus on quality genetics and post-harvest practices, there is hope.

“Brazil has the opportunity now to really turn this horrible page and now introduce not only resistant but productive and good flavour profile varieties,” he says. “That’s the only way I see to survive as a farmer.”

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Written by Zach Latimore.

PDG Cacao

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